Although microcracking of the surface of polymethyl methacrylate (PMMA) articles has been the subject of considerable study, in the past it has not been possible to avoid this phenomenon which results in the appearance after aging of a certain number of small superficial cracks on the surface of the PMMA. These cracks, which can be quite deep, affect not only the appearance of the surface, but sometimes also affect the mechanical strength of the PMMA. This surface cracking is principally caused by humidity, solvents, and certain corrosive atmospheres when the PMMA is under stress. The surface cracking, which may even entail the destruction of the PMMA layer, is sometimes due to internal tensions originating from inadequate conversion of the resin leaving some of the monomer present in the polymer.
Because of this disadvantage PMMA, which otherwise possesses excellent properties such as transparency, resistance to ultraviolet rays and X-ray transmission, is not very suitable for use in certain applications where freedom from cracking and strength are important.
Efforts to eliminate this cracking while at the same time retaining the desired properties of the PMMA structure have not, heretofore, been completely successful.